2. A factorization
Let A be an m × n matrix. The factorization of A takes the form:
A = USV T
• U is an m × m orthogonal matrix.
• V is an n × n orthogonal matrix.
• S is an m × n matrix with nonzero element along the diagonal.
• Assuming m ≥ n
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3. Properties
To factorize A, we consider:
• AT A and AAT
Some Properties
• The matrices AT A and AAT are symmetric.
• The eigenvalues of AT A and AAT are:
• Real and Nonnegative
• Nonzero
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4. Proofs
(i) The matrices AT A and AAT are symmetric
AT
A = (AT
A)T
= (A)T
(AT
)T
(1)
= AT
A (2)
So similar is the AAT
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5. Proof: (ii)The eigenvalues of AT A and AAT are real &
nonnegatives
The matrix AT A is symmetric so its eigenvalues are real numbers.
Suppose that v is an eigenvector of AT A with ||v||2 = 1
corresponding to the eigenvalue λ. Then
0 ≤ ||Av||2 = (Av)T
(Av) = vT
AT
Av (3)
= vT
(AT
Av) = vT
(λv) (4)
= λvT
v (5)
= λ||v||2 (6)
= λ (7)
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6. Proof: (iii)The eigenvalues of AT A and AAT are nonzero
Let v be an eigenvector corresponding to nonzero eigenvalue λ of
AT A. Then
AT
Av = λv It also implies (8)
(AAT
)Av = λ(Av) (9)
If Av = 0, then
AT Av = AT 0, this contradicts the assumption that λ 6= 0
Hence Av 6= 0 and Av is an eigenvector of AAT associated with λ.
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7. Example 1
Determine the singular values of the 5 x 3 matrix
A =
1 0 1
0 1 0
0 1 1
0 1 0
1 1 0
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